Polymer films possessing multi-functional properties, such as transparency, anti-reflectivity, superhydrophobicity and self-cleaning properties, have many important applications ranging from small digital micro-fluid devices and precise optical components to large implementations such as display screen, solar panels and building materials. Generally transparency and superhydrophocity are two competitive properties. Superhydrophobicity and the derived self-cleaning properties use hierarchical fine structures with high surface roughness. However, the high roughness can cause significant light scattering that reduces transparency. By controlling the surface roughness to be less than 100 nm and maintaining a high ratio of air to solid interface, superhydrophobicity and transparency in the visible region of the spectrum can be simultaneously achieved. Additionally, in order to simultaneously implement anti-reflective (AR) properties in visible region of the spectrum using surface structures, one must ensure the nanopores on the surface are smaller than the wavelength and arranged in a gradient distribution so that the refractive index of the surface varies gradually from the bulk material to air.
Techniques to prepare such advanced multi-functional surfaces typically involves multisteps, expensive equipment, releasing of toxic chemicals and are limited to small and flat areas. Developing new methods that are low-cost, environmental friendly and compatible with industrial roll-to-roll manufacturing processes to make such multifunctional surfaces would be industrially significant.
Generally, micro/nanofabrication techniques can be divided into two strategies: top-down and bottom-up as shown in FIG. 1A and FIG. 1B, respectively. The top-down method of FIG. 1A, typically utilize specific nanofabrication equipment to etch unprotected materials to create the expected micro or nanoscaled structures. Various lithography methods and other wet or dry etching methods are typical examples of methods used for the top-down strategy. These top-down methods require expensive process tools, are limited to small size samples and can waste valuable materials during etching. Bottom-up methods, such as the method illustrated in FIG. 1B, often involve methods that directly grow, deposit or assemble nanoscale materials such as nanoparticles, fibers or tubes onto substrates. One significant problem with the bottom-up methods is that organic solvents or noxious and expensive chemicals are used, wasted and subsequently released into environment during the fabrication process. Sample size and throughput is typically limited to small samples. Therefore, an improved method is desired.